Guard-L stands for Guard Lancer which separates it from Guard-S and Guard-A due to wielding different weapon.
Lore[]
Guards are people who protect citizens and bring order. They are armed and wearing protective gear. You can meet them in villagers and guarding garrisons.
Guards are human enemies that you will face if you follow the path of evil and have low moral score. There are 3 specific types: Guard-A, Guard-S and Guard-L.
First time when you have to face guards in a fight is during Kill Tancred quest. Majority of fight you will face them during Battle Of Darsov.
Strategy[]
What makes them different than Guard-S is ability to kneel down and lacking a shield. They start swinging lancer the moment they starts eye contact you. You can block a lancer attack with the shield.